Photosensitive resin structures for flexographic printing plates are, as described in Patent Documents 1, 2 and 3, for example, generally ones in which a photosensitive layer composed of a photosensitive resin composition containing a thermoplastic elastomer, at least one photopolymerizable unsaturated compound and at least one initiator sensitive to radiation is provided on a support such as a polyester film. Further optionally, for the purpose of smoothing the contact with a negative film, a slipping layer, a protective layer or a thin layer referred to as an ultraviolet blocking layer containing an infrared-sensitive substance which allows to be cut off by an infrared laser is often provided on the photosensitive layer. Making flexographic printing plates from such photosensitive resin structures for flexographic printing plates generally involves: first subjecting the whole surface of such a resin structure to ultraviolet exposure through a support (back exposure) to provide a thin uniform cured layer; then subjecting the surface of a photosensitive resin layer to image exposure by ultraviolet rays (relief exposure) through a negative film or directly on an ultraviolet blocking layer; washing out unexposed parts with a solvent for development, or making an absorbing layer absorb and remove unexposed parts after heating and melting; and obtaining desired images, i.e. relief images to make printing plates.
The printing system using a photosensitive resin plate for flexography is one in which an ink is supplied on the surfaces of convex parts of the resin plate having concaves and convexes by an ink-supplying roll, etc., and the resin plate is then made to contact with a substrates to transfer the ink on the convex surfaces to the material to be printed on.
Such a flexography has a problem that a desired printed matter cannot be obtained if the image reproducibility decreases, namely, if a favorable convex line (fine line) shoulder shape and sufficiently deep concave lines cannot be obtained.
Further, the flexography has a problem that plate relief parts chip off because the plate contacts many times with an ink supplying roll and substrates, and because ink stains accumulated on the printing plate are wiped off with a cloth, etc. to remove the stains.
Since when the thickness of a printing plate is uneven, deformations in images are generated because parts where ink has not been transferred are generated and because the pressure at printing becomes excessive, the thickness is desirably as uniform as possible. Since if a photosensitive resin structure generates cold flow during storage and transport, the thickness partially decreases, it preferably has a cold flow resistance as high as possible.
As such photosensitive resin structures, those described in Patent Documents 1, 2 and 3 are known, any of which uses a polymer of butadiene of from several hundreds to less than 10,000 in number-average molecular weight (Mn) as a plasticizer.
Patent Document 4 describes a photosensitive elastomer composition containing a high-molecular weight elastomer diene polymer and a low-molecular weight diene polymer of about 1,000 to 25,000 in molecular weight Mn. It specifically discloses a polyisoprene of 16,000 in molecular weight and a polybutadiene of not more than 4,000 in molecular weight.
Patent Document 5 describes a photo-curable polymer composition containing a low-molecular weight block copolymer (c) of 1,000 to 35,000 in weight-average molecular weight having one block A derived from a polymerizable monovinyl aromatic monomer and at least one block B derived from a polymerizable conjugated diene monomer. Besides that, Patent Document 5 exemplifies a polybutadiene as a commonly used plasticizer, whose molecular weight is 300 to 35,000. However, it describes that since the low-molecular weight block copolymer (c) functions as a plasticizer, not using the plasticizer is preferable, and examples therein use only a styrene-isoprene copolymer of 30,000 in molecular weight as the block copolymer (c).
Further, although Patent Document 6 proposes addition of an isoprene polymer of not less than 25,000 in molecular weight as a plasticizer for improving the solvent resistance of a flexographic printing plate, isoprene does not always provide a sufficient printing durability, a favorable image reproducibility of a fine line relief shape and reverse fine lines, and excellent printed matters.
As described above, in conventional arts, no examples using a liquid polybutadiene which is highly polymerized to not less than 10,000 and which has 1,2-vinyl of not more than a specific content or a sharp molecular weight distribution for a photosensitive resin composition for flexography have been known. A problem of improving both the cold flow resistance and the durability (chipping resistance) of printing plates at printing, and the image reproducibility, which are mutually conflicting effects, has not also been solved so far.    Patent Document 1: JP-A-2000-155418    Patent Document 2: JP-A-2000-181060    Patent Document 3: JP-A-11-153865    Patent Document 4: JP-B-06-039547    Patent Document 5: JP-A-2002-519465    Patent Document 6: Japanese Patent No. 2635461